What if we made the front face (baffle) out of 2 ft x 2 ft acoustical ceiling tile? What is impact on sound? I was just thinking this could be a very low-visibility ceiling or wall mounted Cornu. If you covered the driver with white speaker grill cloth it would be almost invisible.

Given that they're porous absorbers, you would find your horn performing quite poorly, most likely.

Yes, but notice how much people try to add porous dampening materials like felt wool, carpet padding, etc to the inside of cabinets. I think the dampening can be tailored by painting with a nonporous paint over the sections that you don't want as much dampening. This stuff is very dense, not like felt or foam but closer to fiberboard. It has the benefit of reducing reflections from the front, and the Cornu is a speaker with a very large baffle area relative to other speaker designs, exceeded maybe by OB's. In fact, one of the complaints about open baffle designs is that they have such a large surface area of reflection in the room. For the Cornu especially, people seem to getting too much peaky bass behavior and this may be beneficial because there is too much bass.

Cal,
What do you mean by ceiling cavity causing too low of a bass cutoff? What about this idea: put the cornu into the false ceiling with only the 24x24 inch ceiling tile face exposed but redirect the horns through slots cut out of ceiling tile? You would have a nice sounding ceiling speaker that is almost invisible. This would mean that cornu would be about 20 inches with say 2 inch wide slots to redirect downwards. The neat thing is that it would be easy to make and try out. I'm sure it would sound better than most ceiling speakers.

Just make it as a regular 24" square, use plywood to mount the drivers, use the ceiling tile and whatever grille cloth over that to disguise it as a regular panel and use the ceiling cavity to disperse the bass.

My concern is that if the bass from the horns is now up in the cavity above the rest of the ceiling tiles that we may need to remove the stuffing from the cornu horns to ensure they don't sound like any other ceiling speakers.

Don Hills,
Thanks for the tip about Horst Moller's creations. Ok, so I am onto something... The other advantage of ceiling tile is that it can be cut with a utility knife or a jigsaw cuts it like butter. You may also readily find it for free at building renovation dumpsters

I see that many of the horns on the HM website show internal passages built or lined with ceiling tile, he says it is used to absorb unwanted stuff above 100 Hz.

Just make it as a regular 24" square, use plywood to mount the drivers, use the ceiling tile and whatever grille cloth over that to disguise it as a regular panel and use the ceiling cavity to disperse the bass.

My concern is that if the bass from the horns is now up in the cavity above the rest of the ceiling tiles that we may need to remove the stuffing from the cornu horns to ensure they don't sound like any other ceiling speakers.

I would redirect horn outputs by making them turn 90 deg towards face so it exits into main room and not false ceiling cavity above tiles.